al difference of latitude, meridional
difference of latitude and the difference of longitude. Reduce real
difference of latitude and difference of longitude to minutes. Take log
of the difference of longitude (Table 42) and add 10. From this log
subtract the log of difference of meridional parts. The result will be
the log tan of the True Course, which find in Table 44. On the same page
find the log sec of true course. Add to this the log of the real
difference of latitude, and if the result is more than 10, subtract 10.
This result will be the log of the distance sailed. This method should
be used only when steaming approximately a North and South course.

Note.--For detailed explanation of Tables 42 and 44 see Bowditch, pp.
271-276.

Assign for Night Reading Arts, in Bowditch:
183-184-185-186-187-188-189-194-259-260-261-262-263-264-265-266-267-268.

Also, one of the examples of Mercator sailing to be done by both the
Inspection and Logarithmic method.

SATURDAY LECTURE

GREAT CIRCLE SAILING--THE CHRONOMETER

In Tuesday's Lecture of this week, I explained how a Great Circle track
was laid down on one of the Great Circle Sailing Charts which are
prepared by the Hydrographic Office.

Supposing, however, you do not have these charts on hand. There is an
easy way to construct a great circle track yourself. Turn to Art. 194,
page 82, in Bowditch. Here is a table with an explanation as to how to
use it. Take, for instance, the same two points between which you just
drew a line on the great circle track. Find the center of this line and
the latitude of that point. At this point draw a line perpendicular to
the course to be sailed, the other end of which must intersect the
corresponding parallel of latitude given in the table. With this point
as the center of a circle, sweep an arc which will intersect the point
left and the point sought. This arc will be the great circle track to
follow.

To find the courses to be sailed, get the difference between the course
at starting and that at the middle of the circle, and find how many
quarter points are contained in it. Now divide the distance from the
starting point to the middle of the circle by the number of quarter
points. That will give the number of miles to sail on each quarter point
course. See this illustration:

[Illustration]

Difference between ENE and E = 2 pts. = 8 quarter points. Say distance
is 1600 miles measured by dividers or secured by Mercator